Polyamide films are widely used as base films for packaging by virtue of the excellent toughness, heat resistance, cold resistance, transparency, printability and chemical resistance. In the use as the base films for packaging, there is an increasing demand in recent years for materials having high heat resistance and excellent dimensional stabilities. For example, in the field of food packaging, the sterilizing method tends to be sophisticated year by year, and as is observed in the retort or high retort foodstuffs, foods are first put in packaging bags and then sterilized at a high temperature for a long period of time. Therefore, the base films are required not to undergo a deformation such as shrinkage or expansion at high temperatures.
The heat resistance of plastics includes the one represented by a heat distortion temperature in a short period of time and the one represented by heat resistance for a long period of time (UL temperature index). In the case of sterilization of the food packaging in the above-mentioned application, the heat distortion temperature in a short period of time is critical.
This heat distortion temperature is measured by a method wherein a material is gradually heated under a predetermined load, whereby the temperature at which the material starts to undergo a distortion is taken as an index for heat resistance. As a standard evaluation method, ASTM D648 or ASTM D1637 may be mentioned. Between the two methods, ASTM D1637 whereby a load for elongation is exerted to the material, is more suitable for the determination of the heat distortion temperature of films. [In order to meet the severe requirements of the market, the present inventors have modified the method of ASTM D1637 to some extent, so that the temperature at which a distortion of .+-.1% (.+-.2% in ASTM D1637) takes place, is taken as the heat distortion initiation temperature and is used as an index for the evaluation of the heat resistance (the details will be given hereinafter)].
As general methods for improving the heat resistance of plastics, it is known (i) to incorporate inorganic fillers, glass fibers or carbon fibers to a crystalline polymer, (ii) to add copper ions to a polyamide (effective for heat stabilization), (iii) to blend a polymer having high heat resistance, and (iv) to introduce heat resistant functional groups to the main polymer chains. However, by the method (i), it is impossible to obtain a heat resistant film having a smooth surface and good transparency. The method (ii) is effective for the improvement of the heat resistance for a long period of time, but is not so effective for the heat distortion temperature in a short period of time. The present inventors consider that the method (iii) of blending a polymer having high heat resistance and the method (iv) of introducing heat resistant functional groups to the main polymer chains, are effective for presenting polyamide films which are useful as highly heat resistant inexpensive packaging base materials.
It is an object of the present invention to provide inexpensive and highly heat resistant packaging base films. However, there is a general belief that a highly heat resistant polymer is highly crystalline, and it is difficult to biaxially stretch it. In fact, a sheet obtained by quenching a molten resin, has white turbidity, and fine crystals are present in the sheet. Since such fine crystals serve as the starting points for rupture during stretching, it is impossible to obtain a biaxially stretched film continuously or constantly. In order to obtain an inexpensive and highly heat resistant film, it is important to select a polymer having a good cost performance (heat resistance/price), and at the same time, it is important to adopt a method whereby the film can be produced continuously at a high speed by biaxial stretching.